Data in a relational database is logically organized in tables, which consist of horizontal rows and vertical columns. Columns are identified by a column-name. Each row in a table contains data pertaining to a specific entry in the database. Each field, defined by the intersection of a row and a column, contains a single item of data.
Each row in a table must have the same set of data items (one for each column in the table), but not all the items need to be filled in. A column can have a default value defined (either as part of the column specification itself or by using a domain with a default value) and this is stored in a field where an explicit value for the data item has not been specified.
If no default value been defined for a column, the NULL value is stored when no data value is supplied (the way the NULL value is displayed depends on the application - in Mimer BSQL the minus sign is used).
A relational database is built up of several inter-dependent tables which can be joined together. Tables are joined by using related values that appear in one or more columns in each of the tables.
Part of the flexibility of a relational database structure is the ability to add more tables to an existing database. A new table can relate to an existing database structure by having columns with data that relates to the data in columns of the existing tables. No alterations to the existing data structure are required.
All the fields in any one column contain the same type of information and are of the same physical length. This length and type of information is defined by a data type, see Data Types in SQL Statements for a detailed description of data types.
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